Synthetic polyalpha-olefins (PAO) have found wide acceptability and commercial success in the lubricant field for their superiority to mineral oil based lubricants. In terms of lubricant property improvement, industrial research effort on synthetic lubricants has led to PAO fluids exhibiting useful viscosities over a wide range of temperature, i.e., improved viscosity index (VI), while also showing lubricity, thermal and oxidative stability and pour point equal to or better than mineral oil. These relatively new synthetic lubricants lower mechanical friction, enhancing mechanical efficiency over the full spectrum of mechanical loads and do so over a wider range of operating conditions than mineral oil. The PAO's are prepared by the polymerization of 1-alkenes using typically Lewis acid or Ziegler catalysts. Their preparation and properties are described by J. Brennan in Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, pp 2-6, incorporated herein by reference in its entirety. PAO incorporating improved lubricant properties are also described by J. A. Brennan in U.S. Pat. Nos. 3,382,291, 3,742,082, and 3,769,363.
In accordance with customary practice in the lubricant arts, PAO's have been blended with a variety of additives such as functional chemicals, oligomers and high polymers and other synthetic and mineral oil based lubricants to confer or improve upon lubricant properties necessary for applications such as engine lubricants, hydraulic fluids, gear lubricants, etc. Blends and their additive components are described in Kirk-Othmer Encyclopedia of Chemical Technology, third edition, volume 14, pages 477-526, incorporated herein in its entirety by reference. A particular goal in the formulation of blends is the enhancement of viscosity index (VI) by the addition of VI improvers which are typically high molecular weight synthetic organic molecules. Such additives are commonly produced from polyisobutylenes, polymethacrylates and polyalkylstyrenes, and used in the molecular weight range of about 45,000 to about 1,700,000. While effective in improving viscosity index, these VI improvers have been found to be deficient in that the very property of high molecular weight that makes them useful as VI improvers also confers upon the blend a vulnerability in shear stability during actual applications. This deficiency dramatically reduces the range of usefulness applications for many VI improver additives. VI enhancers more frequently used are high molecular weight acrylics. Their usefulness is further compromised by cost since they are relatively expensive polymeric substances that may constitute a significant proportion of the final lubricant blend. Accordingly, workers in the lubricant arts continue to search for additives to produce better lubricant blends with high viscosity index. However, VI improvers and lubricant mixtures containing VI improvers are preferred that are less vulnerable to viscosity degradation by shearing forces in actual applications. Preferred liquids are those that exhibit Newtonian behavior under conditions of high temperature and high shear rate, i.e., viscosities which are independent of shear rate. To the extent that such sought after shear stable fluids retain viscosity under high shear stress at high temperature they would provide a significant advantage over conventional mineral oil lubricants or prior art synthetic hydrocarbon (PAO) lubricants. The advantage would be readily demonstrated in applications such as internal combustion engines where the use of a shear stable lubricant under the high temperature, high shear conditions found therein would result in less engine wear and longer engine life. These fluids must also retain, or improve upon, other important properties of successful commercial lubricants such as thermal and oxidative stability.
Recently, novel lubricant compositions (referred to herein as HVI--PAO and the HVI--PAO process) comprising polyalpha-olefins and methods for their preparation employing as catalyst reduced chromium on a silica support have been disclosed in U.S. patent applications Ser. Nos. 210,434 and 210,435 filed June 23, 1988, now U.S. Pat. Nos. 4,827,064 and 4,827,023, incorporated herein by reference in their entirety. The process comprises contacting C.sub.6 -C.sub.20 1-alkene feedstock with reduced valence state chromium oxide catalyst on porous silica support under oligomerizing conditions in an oligomerization zone whereby high viscosity, high VI liquid hydrocarbon lubricant is produced having branch ratios of less than 0.19 and pour point below -15.degree. C. The process is distinctive in that little isomerization of the olefinic bond occurs compared to known oligomerization methods to produce polyalpha-olefins using Lewis acid catalyst. Their very unique structure provides opportunities for the formulation of superior lubricant blends.
Accordingly, it is an object of the present invention to provide novel, high viscosity lubricant compositions having improved viscosity index and shear stability from alpha-olefins that can be utilized as lubricant VI improver additives.
It is a further object of the present invention to provide novel lubricant basestock blends from high viscosity, high viscosity index HVI--PAO in conjunction with synthetic and natural petroleum lubricant.
Another object of the present invention is to provide novel lubricant compositions from high viscosity, high viscosity index PAO blends with mineral oil and/or conventional PAO lubricants whereby blends with superior viscosity indices and high temperature shear stability are produced.
Yet another object of the present invention is to provide high VI automotive engine lubricating oils that show Newtonian behavior under conditions of high temperature, high shear stress.